Articles | Volume 15, issue 10
Atmos. Chem. Phys., 15, 5903–5928, 2015
Atmos. Chem. Phys., 15, 5903–5928, 2015

Research article 28 May 2015

Research article | 28 May 2015

A multi-model evaluation of aerosols over South Asia: common problems and possible causes

X. Pan1, M. Chin1, R. Gautam2, H. Bian1,3, D. Kim1,4, P. R. Colarco1, T. L. Diehl1,4,*, T. Takemura5, L. Pozzoli6, K. Tsigaridis7,8, S. Bauer7,8, and N. Bellouin9 X. Pan et al.
  • 1NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 2Centre of Studies in Resources Engineering and Interdisciplinary Program in Climate Studies, Indian Institute of Technology, Bombay, Mumbai, India
  • 3Joint Center for Earth Systems Technology, University of Maryland Baltimore City, Baltimore, MD, USA
  • 4Universities Space Research Association, Columbia, MD, USA
  • 5Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan
  • 6Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey
  • 7NASA, Goddard Institute for Space Studies, New York, NY, USA
  • 8Center for Climate Systems Research, Columbia University, New York, NY, USA
  • 9Department of Meteorology, University of Reading, Reading, Berkshire, United Kingdom
  • *current address: European Commission at the Joint Research Center, Ispra, Italy

Abstract. Atmospheric pollution over South Asia attracts special attention due to its effects on regional climate, water cycle and human health. These effects are potentially growing owing to rising trends of anthropogenic aerosol emissions. In this study, the spatio-temporal aerosol distributions over South Asia from seven global aerosol models are evaluated against aerosol retrievals from NASA satellite sensors and ground-based measurements for the period of 2000–2007. Overall, substantial underestimations of aerosol loading over South Asia are found systematically in most model simulations. Averaged over the entire South Asia, the annual mean aerosol optical depth (AOD) is underestimated by a range 15 to 44% across models compared to MISR (Multi-angle Imaging SpectroRadiometer), which is the lowest bound among various satellite AOD retrievals (from MISR, SeaWiFS (Sea-Viewing Wide Field-of-View Sensor), MODIS (Moderate Resolution Imaging Spectroradiometer) Aqua and Terra). In particular during the post-monsoon and wintertime periods (i.e., October–January), when agricultural waste burning and anthropogenic emissions dominate, models fail to capture AOD and aerosol absorption optical depth (AAOD) over the Indo–Gangetic Plain (IGP) compared to ground-based Aerosol Robotic Network (AERONET) sunphotometer measurements. The underestimations of aerosol loading in models generally occur in the lower troposphere (below 2 km) based on the comparisons of aerosol extinction profiles calculated by the models with those from Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) data. Furthermore, surface concentrations of all aerosol components (sulfate, nitrate, organic aerosol (OA) and black carbon (BC)) from the models are found much lower than in situ measurements in winter. Several possible causes for these common problems of underestimating aerosols in models during the post-monsoon and wintertime periods are identified: the aerosol hygroscopic growth and formation of secondary inorganic aerosol are suppressed in the models because relative humidity (RH) is biased far too low in the boundary layer and thus foggy conditions are poorly represented in current models, the nitrate aerosol is either missing or inadequately accounted for, and emissions from agricultural waste burning and biofuel usage are too low in the emission inventories. These common problems and possible causes found in multiple models point out directions for future model improvements in this important region.

Final-revised paper